Hi, Eqn for low pass RC filter (Vout/Vin) = (1/jwc) / ((1/jwc)+R) R = 1khz C=3.1847 x 10^-8 f= 50khz Vin= 2v Find Vout express in Db My working: i find the magnitude of both side which gives vout = 2/ sqrt(1+R^2 w^2 c^2) sub in all values i get vout = 4.997 x 10^-5 change to db i get : 20log(4.997x10^-5) = -86.02db which is wrong answer shld be around 5db what when wrong? Thks. to admin: Sorry for the "Urgent help needed" topic......pls remove it
db = 20 log( vo/vi ) = 20 log( zc / (zc+r) ) = 20 log( (1/wc) / (1/wc+r) ) = 20 log( (1 / (2 pi 50k 3.18e-8) / (1 / (2 pi 50k 3.18e-8) + 1k) ) = 20 log( 100 / (100+1k) ) = -20 db
This calculation actually comes out to -20.83dB, which is wrong. You forgot that the capacitor's reactance is 90 degrees out of phase with the resistance. You have to find the vector magnitude of the denominator. Gain = 20 log( (1/wc) / sqrt((R^2) + (1/wc)^2) Gain = -20.05dB The OP wanted Vout when Vin=2V. Vout=-14.03dB.
Ron, I got 198.9 mV as Vo, which is approximately -20.05 dB with respect to 2 Vi. I didn't understand your Vo being -14.03 dB
The OP wrote: You got Vout=198.9 mV. 20 log (.1989) = -14.03dB. You are correct in showing gain as -20.05dB, which will of course be true for any input voltage. I realize that dB is a unitless gain measurement (a ratio), but the OP said I suppose to be correct, we should say Vout=-14.03dBV, i.e., relative to 1 volt, which identifies it as an absolute measurement rather than a ratio.
That is true Ron. Had you stated dBV, there wouldn't be an issue. At least you didn't use dBf as the reference ...
Database Format? Divorced Black Female? Decibels (frequency)? I guess I'm not up-to-date on acronyms.
femtowatt. I saw it on an HP signal generator back in the early 80s. Not one of the more popular references.
That's funny! Definitely engineering humor, though. My wife doesn't think much of any of my jokes or "funny" stories. I guess I won't try that one on her.
If I didn't have to look it up, I would have never remembered it. It was on the old HP-8640 signal generator series. And to think 1 uV (50 ohms) is 130 dBf. The stuff people remember is amazing sometimes.